1. Technical Field
The present invention is directed toward liquid delivery devices for controlling the flow of liquid from a liquid reservoir, and more particularly toward an improved accuracy peristaltic pump.
2. Background Art
Peristaltic pumps are particularly suited for use in accurately metering and infusing fluids such as medications into the bodies of hospital patients. Heminway, U.S. Pat. No. 4,893,991, discloses a linear type of peristaltic pump. Natwick, U.S. Pat. No. 5,055,001, discloses a valve/plunger type of peristaltic pump. Galea, U.S. Pat. No. 3,999,891, discloses a rotary type peristaltic pump. Each of these peristaltic pumps operate to propel liquid through a resilient tubing which is typically made from a flexible plastic material such as polyvinyl chloride or the like. The tubing is repeatedly compressed and expanded along a defined section of the tubing. The tube is typically expanded or "rebounded" by its internal resiliency. A known problem with this type of pump is that the portions of tubing which are intermittently compressed and expanded tend to fatigue with time. As a result, the tubing is less able to rebound to its original cross-section when released, decreasing the volume of the tubing along the critical pump segment and thereby degrading pump accuracy. U.S. Pat. No. 4,893,991 notes that such pumps have been found to exhibit as much as a 10% drop in flow rate in a 24 hour period.
The prior art has recognized this short coming in peristaltic pumps and has attempted at least three ways to solve the problem. First, Heminway, U.S. Pat. No. 4,893,991, attempts to improve pump accuracy by preventing the portion of resilient tubing which is subject to compression and expansion from assuming a cylindrical configuration upon expansion. That is, the plungers which compress and expand the tubing are designed to maintain the segment of tubing in an oval cross-section even at full expansion. A principal problem with the solution set forth in Heminway is that it requires very accurate tolerances with the plungers in the retracted position so that the tubing expands to a consistent oval cross-section in order for the pump to operate at accurate rates and volumes. In addition, because the tubing is not able to assume its full circular cross-section, and therefore its greatest volume, Heminway unduly restricts the rate liquid can be pumped.
Natwick, U.S. Pat. No. 5,055,001, proposes an even more complicated solution to improving accuracy in peristaltic pumps. Natwick proposes that the range of diametric compression of the tubing be from about 15% with the plunger retracted to about 85% with the plunger extended. Natwick argues that since the tubing need never recover to a fully uncompressed condition, changes in the elasticity of the flexible tubing due to continued use and repeated compression have much less effect on the volumetric capacity of the pump. In addition, because the plunger never fully compresses the pumping portion of the tubing, the tubing is subjected to less fatigue. Natwick further teaches providing mechanical tubing shapers disposed on each side of the plunger which are extended to reform the pumping portion of the tubing as the plunger is retracted and the tubing refills with fluid. Natwick suffers from the same shortfall of Heminway in that it restricts the volume of the tubing used for pumping and therefore limits pump output rates. In addition, the tubing shapers are complex mechanical structures which create an additional avenue for potential pump failure. Moreover, the mechanical shapers taught in Natwick require a number of potentially costly parts and complicate the assembly of the pump.
Mannes, U.S. Pat. No. 4,585,442, discloses an intravenous infusion rate controller which operates on a resilient tubing which rests in a trough between a pair of resilient bands. The resilient bands act on opposing sides of the outer diameter of the tubing in a compressed state to aid in restoring the tube to its original cross-section upon expansion. In this manner the resilient bands inhibit the tendency of the tube to "flatten out" and rebound to only an oval cross-section which degrades the accuracy of the rate controller. Unfortunately, the resilient bands act on only two discrete points in attempting to restore the tubing to its original shape. Moreover, the space between the resilient bands must be maintained at rather precise tolerances to avoid the resilient bands compressing the tubing into an oval cross-section if the bands are too close or failing to restore the tube to its circular cross-section if the bands are too far apart.
The present invention is directed toward overcoming one or more of the problems discussed above.